A common manufacturing process for acetic acid and related alkyl carboxylic acids is the reaction of a solution of an alcohol of the appropriate carbon skeleton with carbon monoxide in the presence of water, the carboxylic acid, a soluble rhodium salt catalyst, and iodine or iodine containing compound as a reaction promoter preferentially an alkyl iodide of the appropriate carbon skeleton. Examples of this process are described by R. T. Eby and T. C. Singleton in Applied Industrial Catalysis, Vol. 1; Academic Press (1983); and U.S. Pat. No. 3,769,329 to Paulik et al.
The catalyst for this process, a soluble rhodium compound, is self-regenerating and is recycled back into the reaction vessel after separation of the product by, for example, a flash distillation or by other similar separation methods.
Due to the corrosive nature of the catalyst solution with operation over an extended period of time, the catalyst solution becomes contaminated with corrosion metal salts; i.e., iron, nickel, molybdenum, chromium, and the like. The presence of these corrosion metal salts increases the production of undesireable by-products such as CO.sub.2, H.sub.2 and CH.sub.4 resulting in a lower raw material efficiency. Decrease catalyst stability and activity can also result from contamination with corrosion metals.
Recovery of the rhodium values has been addressed in U.S. Pat. No. 3,887,489, to Monsanto Company, by Fannin et al.
According to that method, the reaction solution, containing the rhodium catalyst, the iodine containing promoter compound, carbon monoxide, and the undesirable corrosion metal salts, is heated at a temperature of from about 100.degree. C. to about 190.degree. C. at a pressure sufficient to boil off all remaining carbon monoxide for a time sufficient to precipitate the rhodium as an insoluble iodide salt. The patent also describes adding the appropriate alcohol prior to the heating step to convert any hydriodic acid to the corresponding alkyl iodide compound and recovering the alkyl iodide by distillation and condensation during heating. This alkyl iodide can then be reused in the catalyst systems. This process leaves the corrosion metal salts in solution. The insoluble rhodium salts are separated from the soluble corrosion metal salts by decantation, filtration, or the like, and then the rhodium is redissolved by addition of suitable solvents, as for example, water, acetic acid, or a water-acetic acid mixture, and saturating the resulting solution with carbon monoxide.
We have found that this described process is operable in a reaction system that has low concentration of iodide ion, mainly as hydriodic acid as in the traditional methanol carbonylation technology. (U.S. Pat. No. 3,769,329 by Paulik et al.) It does not work in the presence of iodide salts such as alkali metal iodides (e.g. LiI) in significant concentrations and in reaction zones where the water content of the reaction mixture is of the order of 5% or less by weight.